We are utilizing the Chince Hamster Ovary (CHO) fibroblast grown in vitro to study the genetics and biochemistry of some aspects of the behavior of cultured cells. Our work has emphasized cell adhesion, morphology, response to cyclic AMP, and the role of cell surface antigens and receptors in cell behavior. We have isolated a variety of different mutants with abnormal adhesion, abnormal morphology or abnormal response to cyclic AMP and some of these have been characterized as to their biochemical defects. We are currently using specific antisera and complement to isolate CHO mutants which have lost specific cell surface antigens or are unable to secrete secific antigens and are establishing general procedures for the isolation of mutants lacking specific cell surface receptors.